This invention relates to a device for terminating the rotary movement of a driven shaft after an adjustable predetermined number of revolutions.
In many areas of technology, devices are required which terminate the rotary movement of a driven shaft following an adjustable predetermined number of revolutions. In most applications, an electric motor drive is involved, the current supplying said motor being interrupted by an electrical signal, produced after a specified number of revolutions of the motor shaft. Preferred areas of application are electrical appliances and positioning drives in designing installations.
The device according to the invention is indeed suitable for all of the applications cited hereinabove, but has been specially developed for electric motor drives for venetian blinds, and will therefore be discussed with reference to this sample application.
In all electric-motor venetian blinds, it is necessary to provide a limit switch internal to the device, which terminates the flow of current after the shaft of the drive motor has made a specified number of revolutions, so that the motor stops. Without such a limit switch internal to the device, there is danger of destruction of the installation by careless actuation of the control device. In the case of horizontal venetian blinds, the supply of current for the selected direction of rotation of the electric-motor drive is interrupted by the limit switch after the slats have been raised by means of the winding shaft. When the venetian blind is lowered, the current is interrupted in similar fashion after the lowest limiting position is reached. In the case of venetian blinds with vertically mounted slats, the drive shaft and/or electric motor must be shut off after the slats of the blind have been collected together at one side, and also after the slats have been spread out laterally, in order to avoid destruction of the installation.
If rotary movement is used to produce a linear displacement, as in the case of venetial blind drives, this type of limit switching can be accomplished in known fashion by limit or stop switches, disposed at the ends of the displacement path. The disadvantage of this known solution consists in the fact that additional wires and switches must be installed.
It is also known to use a counter drive to shut off the rotary movement after a predetermined number of revolutions, said counter drive actuating a microswitch for example. A counter drive of this kind is relatively complex in design and poses difficulties as far as adjusting the desired number of revolutions is concerned.
Finally, devices of the type described hereinabove are known wherein the switch disks, disposed coaxially adjacent to one another and provided with notches in their outer circumference, are driven at slightly different rotational speeds. A switch arm, extending axially above both switch disks, drops into the notches, when the angular position of the latter coincides in the vicinity of the switch arm. As it drops, the switch arm interrupts the supply of current to the drive motor, the arm actuating a microswitch for example. The adjustment of the end positions is accomplished by turning the switch disks on their shaft and then locking them in place using grub screws or notches.
When grub screws are used to lock the disks in position, there is the danger that these screws will be loosened by the vibrations produced by the motor and will therefore not hold the set end positions. Furthermore, the grub screws can deform the shaft if they are tightened excessively, since the shaft, at least in the case of one switch disk, must be made in the form of a hollow shaft. Even when the adjustment disk are held in place by notches, loosening may occur resulting in rotation of the switch disks from their set positions.
Finally, it is disadvantageous that a tool is required to adjust the switch disks.